How to make a list of lists in Python?

Outside of this loop I want a variable,say x to be assigned such that it gives the following output.

['A', 'A', 'A']
['C', 'T', 'C']
['G', 'T', 'C']
['T', 'T', 'C']

After doing this, here is what I should be able to do:
In the Next, I want to compare within x. Say x[2] to tell me if 'G' is different from 'T' and if 'G' is different from 'C' and if 'T' is different from 'C'